Geochemical and petrological investigation into the magmatic system at Soufrière Hills Volcano, Montserrat.
Andesite lavas from the current eruption of the Soufrière Hills Volcano, Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. The processes of mass transfer and flux of material involved in the interaction between mafic and silicic magmas are crucial for understanding eruption dynamics. Detailed textural, petrological and geochemical analyses coupled with field observations are used to define a classification scheme for mafic enclave types in erupted andesite from phase V (2009–2010). Type A are closest to a mafic end-member, whereas type B are significantly hybridised. Type A quench crystallisation is driven by rapid thermal equilibration during injection into the andesite. Type B enclaves form from a slower cooling vesiculating hybridised melt layer. Type C, are a composite of types A and types B, representing an interface between the types. Geochemical modelling shows that since the start of the current eruption that the dominant control on the range of enclave bulk compositions has changed from fractional crystallisation to mixing of the mafic end-member and host andesite. A change in selected elements concentrations in the mafic end-member is observed from phases I to III, halting in phase V. Volatile flux from the mafic to andesite magmas plays a vital role in eruption dynamics, but evidence for vapour transport in the erupted lavas is rarely preserved. Geochemical and petrological evidence is presented for the segregation and transport of metal-bearing vapour in shear fractures generated in the shallow conduit or dome during magma ascent. Elevated metal concentrations (Cu, Au, Ag, Pb, Zn) indicate magmatic vapours transport. Volatiles were resorbed into the partial melt generated during frictional heating (>1000 °C) at the slip surface as a peraluminous partial melt recrystallised. The shear zones provide evidence for degassing and an insight into controls on eruption style.
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